Electrical apparatus for measuring the rate of change of an input voltage



Aug. 21, 1951 L. H. BEDFORD ET AL 2,564,829 ELECTRICAL APPARATUS FOR MEASURING THE RATE OF CHANGE OF AN INPUT VOLTAGE Original Filed Nov. 30, 1943 VARYING A.C.VOLTAGE WHOSl RATE or CHANGE 15 TO BE MEASURED POLARISING VOLTAGE (IF REQUIRED) VARYING A.C. VOLTAGE WHOSE RATE or CHANGE IS'T BE MEASURED POLARISING/ VOLTAGE (1r REQUIRED) INVENTORS Leslie HerbedBed/ord BY Joluo Bell LqcMzZLes [mug/Iain,

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attorney Patented Aug. 21, 1951 ELECTRICAL APPARATUS FOR MEASURING THE RATE OF CHANGE OF AN INPUT VOLTAGE;

Leslie Herbert Bedford, London, and John Bell and Eric Miles Langham, Teddington,

England,

assignors to A. C. Cossor Limited, London, England, a British company Original application November 30, 1943, Serial No. 512,402. Divided and this application July 26, 1946, Serial No. November 30, 1942 686,479. In Great Britain 7 Claims. (Cl. 17195) This invention relates to an electrical circuit for measuring the rate of change of a varying voltage over a period of time. The measurement obtained by the use of the invention is a weighted average value of the rate of change, in which the later values of the rate of change are given greater significance.

The circuit according to the invention, for measuring the rate of change of a varying direct voltage, comprises a pair of condensers each arranged to be charged from said input voltage through resistance, said condensers and resistances being so related that the charging time constants of said condensers are different and means to measure the difference between the voltages across said condensers.

In the preferred arrangement, one of said condensers is connected in series with a resistance across the other of said condensers which in turn is connected in series with another resistance across said varying direct input voltage.

If the circuit is to be applied to the measurement of the rate of change of amplitude of an alternating voltage, this is first rectified. If the amplitude is liable to pass through zero, however, an alternating voltage of the same phase and of constant amplitude must be superposed to prevent this, owing to the inability of the rectifier to give sense indication.

In the accompanyingdrawing, Figure 1 is a, diagram of a circuit arranged for measuring, in accordance with the invention, the rate of change of the amplitude of an alternating voltage applied between input terminal l5 and earth. This may, for example, be an amplitude-modulated 50-cycle voltage.

The voltage between terminal I5 and earth is rectified by diode I, so that a direct voltage proportional to its amplitude is developed across diode load 2 and condenser 3. This direct voltage is applied to two circuits of different time constants, in parallel with each other and comprising respectively condenser 6 in series with resistance 1 and condenser 8 in series with resistance 9. In a typical example, the resistances 1 and 9 may each have the value 1 megohm while the condensers 6 and 8 have respectively the values 4 microfarads and 8 microfarads. If the amplitude of the applied voltage varies linearly for a period of time, the difference between the direct voltages built up across the condensers 6 and '8 of the time constant circuits represents, after a time interval, the value of the rate of change of amplitude. This diiference voltage is developed as a direct output voltage between terminals 4 and 5, neither of which is earthed. The voltage may be meas-' ured by any currentless direct voltage voltmeter. An apparatus such as that described in patent application Serial No. 686,478, may be used for this purpose.

Figure 2 is a fragmentary diagram showing an alternative arrangement of the time constant circuits of Figure l, which has the advantage that one side of the output voltage representing the rate of change is earthed and that inaccuracies due to leakage are minimised. In the arrangement of Figure 2, resistance I0 is common to both time constant circuits, while resistance I2 is included in the circuit of condenser ll only. The difference between the voltages developed across condensers I4 and H, which is a measure of the value of the rate of change, appears between terminals l3 and earth. In a typical example, the values of resistances ll] and I 2 may be 0.27 and 1.0 megohm respectively, while the values of condensers l4 and II may both be 2 microfarads. Since the charging time constant of each condenser is represented by the product of the condenser capacity and the resistance in series with it through which it is charged by the input voltage, the foregoing values give charging time constants for the circuits of condensers I4 and H of .54 and 2.54, respectively; i. e., the charging time constant of condenser I4 is while the charging time constant of condenser I I is in Figures 1 and 2 as earthed at their lower ends, the circuits will function satisfactorily if, instead, the line shown as earthed is maintained at any constant amplitude alternating potential to earth in phase with that of terminal l5.

If the amplitude of the alternating voltage input at terminal I5 relative to earth is liable to pass through zero, the system fails owing to the inability of the rectifier to give sense indication. This difficulty is overcome by superposing upon the varying alternating voltage, the rate of change of amplitude of which is. to be measured, an alternating voltage having the" same phase and having a constant amplitude greater than the maximum amplitude orv said. varying alternating voltage. This may be applied between earth and the line shown as earthed in Figures 1 and 2.

It will be observed that the voltage, the rate of change of which is measured, is actually the varying direct voltage established across the diode load 2. The upper end of this load, connected to the cathode of the diode, is a point at high D. C. resistance to earth, but the lower end, even if it be connected. to an alternating supply line, is at low D, C. resistance to earth. The advantages of the arrangement of Figure 2 are attained in allcases.- where: one side only of the direct voltage is at low D; C. resistance to earth and the junction of condenser M and resistance 12- is connected to this side.

This application is a division of application Serial No. 512/102, filed November 30, 1943.

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1. An electrical measuring apparatus including a load, means for producing a varying direct input voltage: across said: load, and a circuit for measuring the rate of: change of the amplitude of. said. input voltage, said. circuit comprising a first resistancev and: a first condenser connected in series with one another and in parallel with said load, a. secondresistance anda second condenser: in. seriesv/ith one another and with said first resistance but in parallel with said first condenser, and: a voltage measuring means connected across said second resistance.

2. An electrical measuring apparatus according to claim 1: including means providing a low direct current resistance path between one side of said load and earth, and wherein said resistances' and condensers are so arranged that the junction between saidfirst condenser and said second resistance is connected to the side of said load having said" low direct" current resistance path to earth.

3. An electrical measuring apparatus according to claim 1 including a rectifier for deriving said: varying direct input voltage; from an alter- 4 nating voltage of varying amplitude, the rate of change of amplitude of which is to be measured, and means for superposing on said alternating voltage of varying amplitude an alternating voltage of the same phase and of constant amplitude greater than the maximum amplitude of said varying alternating voltage in order to prevent the voltage across said rectifier from falling to zero.

l. An electrical measuring apparatus including a load, means for producing a varying direct input voltage across said load, and a circuit for measuring the rate of change of the amplitude of said input voltage, said circuit comprising a first resistance and a first condenser in series with one another and so connected to said load that said first condenser is charged by said input voltage through said first resistance, a circuit loop in parallel with said first condenser and in series with said first resistance comprising a second resistance and a second condenser in serieswith one another, the values of said resistances and condensers being so related that the charging time constants of said first and second condensers are different, and means for measuring the voltage drop? across: said second resistance, said voltage drop representing a weighted average value of the rate ofchange of the amplitude of. said input voltage.

5. An electrical measuring: apparatus accord.- ing to claim 4" wherein said firstandsecond condensers are of equal capacity.

6. An electrical measuring. apparatusaccording to claim 5 wherein the capacity of. each of said first and second condensers: is: 2 microfarads and the: resistances of said first and second re:-' sistances are .27v megohmand megohm, re spectively.

'7. An electrical measuring apparatus according to claim: 4 wherein the ratio betweenthe charging time constants of saidfirst. and: second condensers, each of which is represented by. the product of the condenser capacity and the re-- sistance in series with it through which: it: is gharsged by said input voltage, is substantially .54

LESLIE: HERBERT BEDFORDZ JOHN BELL. ERIC LANGHAM.

REFERENCES CITED The following references are of record in the file of this patent:-

UNITED STATES: PATENTS Number A Name. Date 2,272,849- Perkins Feb. 10; I942 2,375,775- Evjen May 15;- I945 

